DESCRIPTION: (Applicant's Description) Activity-dependent modulatior of Ca channels may contribute importantly to short-term synaptic plasticity, believed to be essential to neurocomputation. Recently, Ca2+-dependent facilitation of presynaptic Ca channels has been linked to synaptic facilitation at the calyx of Held. Furthermore, Lee et al (1999) reported modest Ca2+-dependent facilitation in recombinant P/Q-type channels, whose native analogs trigger neurotransmitter release at many sites including the calyx of Held. Such facilitation reported involves Ca2+-dependent calmodulin (CaM) binding to a "CBD" site on C-tail of the P/Q channels. Although features of P/Q channels now emerge as molecular correlates to Ca2+-dependent synaptic facilitation, critical issues remain, as addressed in these Aims: (1) To clarify the functional capacity of P/Q Ca channels to undergo Ca2+-dependent facilitation. Unlike the initial report that channel facilitation may be modest, preliminary results suggest that such facilitation is striking, as gauged by protocols designed with appropriate biophysical insight. (2) To elucidate the molecular requirements for CaM to support facilitation of P/Q channels. This aim will exploit coexpression of recombinant CaM with P/Q channels to test specific features of CaM modulation, such as whether CaM is "tethered" to the channel. (3) To delineate structural features of the P/Q-type channel that permit facilitation by Ca2+-activated CaM. We have recently identified an IQ domain on the alpha1A C-tail as another CaM binding site. This aim will investigate whether CBD and/or IQ are in fact crucial for channel facilitation.